This invention relates to a process for the conversion of a hydrocarbon feedstock to produce low sulfur diesel and aromatic compounds including xylenes. More specifically, the invention concerns the selective hydrocracking of multi-ring aromatic compounds contained in the hydrocarbon feedstock to produce low sulfur diesel and aromatic compounds which are transalkylated in an integrated transalkylation zone to produce the most desirable xylene isomers.
It has been recognized that due to environmental concerns and newly enacted rules and regulations, saleable products must meet lower and lower limits on contaminants such as sulfur and nitrogen. Recently new regulations require the essentially complete removal of sulfur from liquid hydrocarbons which are used in transportation fuels, such as gasoline and diesel.
The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is paraxylene, the principal feedstock for polyester which continues to enjoy a high growth rate from a large base demand. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Metaxylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand and conversion of other hydrocarbons as necessary to increase the yield of xylenes and benzene. Most commonly, toluene is dealkylated to produce benzene or disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered. More recently, processes have been introduced to disproportionate toluene selectively to obtain higher-than-equilibrium yields of paraxylene.